Commercial non-impact printing systems typically use a method of developing toner (liquid or dry powder) to an electric or magnetic latent image created by some writing means. Generally associated with the creation of the latent image are an imaging cylinder, some means for creating the image, and associated conditioning means for residual image removal and cleaning. All of these components wear out during system operation and must be added to the cost of each printed page. Toner itself costs somewhere (in 1994) in the neighborhood of $0.0006 to $0.001 per page. Adding in the rest of the consumable components, the cost is raised to a range of $0.0625 to $0.0065 per page. Latent image non-impact printing carries a considerable additional imaging cost. Direct-to-paper imaging systems such as ink jet technologies carry only the cost of the ink; however, many of these technologies do not obtain imaging as desirable or quick or versatile as latent image systems do.
Another technology that is not commercial but attempts to obtain direct-to-paper imaging (that is without a latent image) is the magnetstylus technology, exemplified by U.S. Pat. Nos. 3,816,840, 4,402,000, and 4,464,672. This technology uses a dry, magnetically attractable, electronically conductive toner which forms a connecting path from the primary to the secondary electrode. The "write" condition of the toner is the active electrode condition and extra toner is removed by a magnetic field. Typically inductive charging of the toner for the "write" condition is used, and the secondary electrode uses a dielectric receptor material above it. This technology has not become commercial, however, primarily due to imaging and background removal problems, as well as problems with transferring the toner to a substrate.
Another proposed technology for direct-to-paper imaging is called direct electrostatic printing (DEP), and is exemplified by U.S. Pat. Nos. 4,860,036 and 4,810,604. This technology typically utilizes some sort of a toner conveyor which moves the toner past the primary electrodes formed by multiple apertures, with an electrically insulated base member clad on one side thereof with a continuous conductive layer of metal, and on the opposite side a segmented conductive layer. Toner passes through the apertures into a web which is moving past a stationary backing electrode or shoe which can be connected up to potential sources to either effect printing or cleaning operations. The toner delivery systems in DEP technology leaves much to be desired, and the dual conductive apertures spaced apart from each other by an insulating member are more complex than is desired.
According to the present invention a method and apparatus are provided which are able to achieve direct-to-paper imaging (that is without a latent image) in a simple yet effective manner. The technology of the present invention may be referred to as "field effect imaging". The invention utilizes non-conductive, non-magnetic toner which does not form a connecting path from the primary to secondary electrodes, has the "write" condition when the primary electrode is de-energized, removes extra toner with an electric field, does not use inductive charging of the toner for the "write" condition, and uses simple primary electrodes, typically pin or stylus simple electrodes disposed in an array. In the field effect method only the electrostatic adhesion force dominates in control of the toner on a "secondary electrode" (typically a conductive surface which can be either positively or negatively charged, or grounded, such as a roller with a conductive surface), and imaging is subtractive in nature (that is the toner in the non-image areas is removed by the primary electrodes).
According to one aspect of the present invention, a method of applying a toner image to a moving substrate (typically paper web), using a non-conductive, non-magnetic toner having a 5-20 micron mean particle size, at least a first moving conductive member, and an array of primary electrodes, is provided. The method comprises the steps of substantially consecutively and continuously: (a) Electrically charging the non-conductive, non-magnetic toner having a 5-20 micron mean particle size to a level of at least about 8 micro Coulombs/gram. (b) Bringing the first moving conducting member into operative association with the electrically charged toner from step (a) so that toner particles adhere thereto, forming a layer thereon. (c) Selectively energizing individual primary electrodes from the array of primary electrodes to cause them to apply electric fields to the layer of toner particles in a no-write condition to effect removal of toner particles where the applied electric field exists at a level greater than an electrostatic adhesion force on the toner particles in the layer, the applied electric field times the charge on the toner being greater than Q.sup.2 /(16*.PI.*.epsilon..sub.0 *r.sup.2), where Q is the charge on the toner, .epsilon..sub.0 is the permitivity constant, and r is the toner particle radius; or selectively de-energizing individual primary electrodes from the array of primary electrodes to cause them not to apply electric fields to the layer of toner particles in a write condition, in which the layer of toner particles merely passes past the array of primary electrodes without toner particles being removed from the layer. (d) Transferring the toner particles remaining on the first conductive member after it passes past the array of primary electrodes to the moving substrate. And, (e) fusing the toner particles to the substrate.
Step (c) is typically practiced to apply an electric field of greater than about 1.6 volts/.mu.M when in the no-write condition. Step (c) is typically further practiced so that the magnitude of the electric field applied in the no-write condition is equal to (V.sub.1 -V.sub.2)/D, where V.sub.1 =the electric potential of the primary electrode, V.sub.2 =the electric potential on the first conductive surface, and D=the separation distance between the primary electrode and the first conductive surface, D=about 75-250 microns.
Typically the toner is in an electrostatic fluidized bed during the practice of step (a), such as shown in European published patent application 494454, and the first surface is moved past the fluidized bed in the practice of step (b), and the toner removed in the no-write condition during the practice of step (c) returns to the fluidized bed. Preferably the primary electrodes are pins or styluses, and the first conductive surface is the exterior surface of the first roller. In that case step (d) is practiced by bringing the exterior surface of the first roller into contact with the moving substrate and by applying a transfer electrical force (e.g. using a transfer corona on the opposite side of the moving web of paper from the roller) to the toner on the exterior surface of the first roller to cause the toner to transfer from a first roller to the substrate. Alternatively a second roller may also be provided having a second conductive exterior surface, in which case step (d) may be practiced by electrically transferring the toner from the first roller to the second roller, and then bringing the exterior surface of the second roller into contact with the moving substrate, and by applying a transfer electrical force to the toner on the exterior surface of the second roller to cause the toner to transfer from the second roller to the substrate. Step (c) may be practiced by utilizing the primary electrode disposed between the first and second rollers, or associated with the first roller remote from the second roller. Where two rollers are utilized, premature transfer of toner from the first roller to the second roller may be provided by shielding the rollers from each other remote from the area of closest proximity between them.
Step (c) is typically practiced by electronic switching of the connection of each primary electrode pin or stylus of the array to a source of electrical potential, by controlling electronic switches using a computer. A flow shield may also be provided mounted just "downstream" of the primary electrode array in the direction of movement of the first roller to cause the toner particles removed from the first roller to fall by gravity into the fluidized bed below it.
According to another aspect of the present invention a field effect imaging apparatus is provided which comprises the following elements: An electrostatic fluidized bed of non-conductive, non-magnetic toner particles. Means for mounting a moving substrate on which toner is to be applied. Means for electrically charging toner particles in the fluidized bed. A first roller having a conductive outer surface mounted for rotation adjacent the fluidized bed to receive charged toner particles from the fluidized bed in a layer on the surface thereof. An array of primary electrodes. Means for selectively applying electrical potential, or no electrical potential, to the individual primary electrodes depending upon whether a no-write or write condition is the exist. And, means for transferring toner from the first roller to a moving substrate mounted by the means for mounting a moving substrate.
The array preferably comprises an array of pin or stylus electrodes and the array may either be mounted adjacent but spaced from the first roller and between the fluidized bed and the substrate (in which case the toner transferring means transfers toner from the first roller directly to the moving substrate), or a second roller may be provided between the first roller and the substrate. In this case the primary electrodes may either be associated with the first electrode, or may be disposed between the rollers so that only the "write" toner is transferred from the first roller to the second roller.
The array pins or styluses may be mounted so that they are spaced about 75-250 microns from the first roller, or from between the rollers. A flow shield for causing toner removed by the no-write conditions of the primary electrodes to fall back into the fluidized bed may be provided as well as a shield between the first and second rollers. The means for electrically charging toner particles in the fluidized bed may be a rotating cylinder with a plurality of corona points, or a corona wire, immersed in the fluidized bed.
According to another aspect of the present invention a field effect imaging apparatus is provided comprising the following elements: Means for mounting a moving substrate. A source of charged toner particles. A first roller having a conductive outer surface mounted for rotation adjacent the source to receive charged toner particles from the source in a layer on the surface thereof. An array of pin or stylus primary electrodes. Means for selectively applying electrical potential, or no electrical potential, to the individual pin or stylus primary electrodes depending upon whether a no-write or write condition is the exist. And, means for transferring toner from the first roller to a moving substrate mounted by the means for mounting a moving substrate.
The first roller conductive exterior surface may be coated with or comprise a conductive hard metal coating; for example it may be coated with hard chrome, tungsten carbide, silicon carbide, or Diamond-Like Nanocomposite.
It is the primary object of the present invention to provide a simple yet effective direct-to-paper imaging system and method. The "direct writing" field effect toning method and apparatus of the invention have no latent image to deal with, the rollers utilized are conductive with hardened surfaces that need no particular conditioning, the imaging (primary) electrode array contains no wearing parts and is not in contact with any moving surfaces, and in general the only consumable is the toner itself. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.